Recent advances in color copying and printing have put increasing importance on developing new methods to prevent forgery of security documents such as banknotes. While there have been many techniques developed, one area of increasing interest is in developing reflective features that cannot be readily reproduced, particularly by a color copier or printer.
One approach that has been taken is to formulate an ink for creating a printed image that is visually distinct from its reproduction. For example, U.S. Pat. Nos. 5,059,245, 5,569,535, and 4,434,010, the entireties of which are incorporated herein by reference, describe the use of stacked thin film platelets or flakes. Images produced with these pigments exhibit angular metamerism. These pigments have been incorporated into security inks used, for example, in paper currency. These pigments have also been incorporated into plastics applications (see, for example, PCT Publication WO 00/24580, published May 4, 2000). Additional inks and reflective features are described in U.S. Pat. Nos. 4,705,356; 4,779,898; 5,278,590; 5,766,738; and 6,114,018, the entireties of which are incorporated herein by reference.
Another approach used to produce security documents has been to produce a “covert” image that contains a material which cannot be seen by the naked eye but which can be made visible under specific conditions. For example, U.S. Pat. Nos. 5,324,567, 5,718,754, and 5,853,464 disclose the use of Raman active compounds. U.S. Pat. Nos. 5,944,881 and 5,980,593 describe fluorescent materials that can be used in an ink. Also, U.S. Pat. No. 4,504,084 discloses a document containing an information marking comprised of a first color that is at least partially opaque or visible in infrared light and a second color, which conceals the first color in the visible spectrum, but is invisible to infrared light.
While these efforts afford printed images that are difficult to reproduce, advances in color copiers and color printers continue to be made. Therefore, the need exists for new highly secure features and for methods for producing such features, particularly for security documents, which features cannot be easily reproduced, and which are visually distinct from their reproductions.
Additionally, the need exists for providing the ability to create reflective features that display variable information, e.g., information that is individualized for a specific product unit, such as a serial number, which variable information cannot be easily or readily duplicated or copied.
The need also exists for reflective features that are highly reflective. Highly reflective features, particularly reflective features that display variable information, are generally more difficult to reproduce than non-reflective features.
The need also exists for highly durable reflective features that can withstand the rigors of use, for example, the extensive handling involved with widespread circulation, or the repeated washing to which authenticated garments may be subject.